The present invention relates to a method for manufacturing cement clinker by which method cement raw meal is calcined in a calciner comprising an upper zone and a lower zone, where fuel, combustion gas and raw meal are introduced into the upper zone of the calciner and directed downward through the calciner, where the combustion gas and raw meal are introduced tangentially into the calciner and subject to rotation follow a spiral-shaped flow path, and where, under the action of gravity, the raw meal is primarily directed down along the wall of the calciner. The invention also relates to a plant for carrying out the method.
A plant of the above-mentioned kind for manufacturing cement is well known from the literature.
A kiln plant of the Separate Line Calcinator Downdraft type (SLC-D) is known for example from the U.S. Pat. No. 4,014,641 and WO 97/30003. This type of plant comprises a calciner, which is configured as a chamber which at its lower end is connected to an upward directed gas duct which is also connected to the outlet end of the kiln for exhaust gases. Also known from international patent application No. PCT/IB01/01296 is a kiln plant of the In Line Calcinator Downdraft type (ILC-D) where the calciner consists in the main of one big chamber. It is a common feature for the two types of plant that most of the fuel used in the calciner is injected axially at the top of the calciner. Preheated air from the clinker cooler and preheated raw meal are introduced tangentially into the top of the calciner. In the kiln plant of the type ILC-D, the exhaust gases from the kiln are also introduced tangentially into the top of the calciner, typically at a location above the point at which the preheated air is introduced in order to establish a NOx-reducing zone at the uppermost part of the calciner. In both types of plant the fuel is thus burned primarily in a zone around the centre-line of the calciner, while, due to the tangential method of introduction, the raw meal and combustion air will flow downward through the calciner along a spiral-shaped path, thereby thrusting the raw meal towards the calciner wall, causing it to slide down along the wall under the action of gravity. As a consequence hereof, simultaneously with the calcination process, the raw meal will effectively protect the lining in the calciner. Advantages of these known types of plant are that fuels with a low content of volatile constituents can be used and that formation of NOx is at a relatively low level. However, it is a distinct disadvantage of such kiln plants that optimum calcination of the raw meal discharged from the calciner has not been achieved. Also, the exhaust gases exiting the calciner may contain unburned fuel, of which a major part will be in the form of CO, which will be ascribable to the fact that the combustion air is not mixed with the fuel to the extent required to provide the necessary air surplus across the entire cross-sectional area of the calciner.